The M-4 Carbine

MAJ Zeski - I am going to assume you are speaking of Terminal Ballistics.  One very important factor to look at is the barrel lengths between the M4 and the M16.  The M4's 14.5in barrel will give us an average muzzle velocity of 2970 feet per second and the M16's 20 inch barrel will give us an average of 3100 feet per second.  Assuming we are shooting two IDENTICAL 5.56 cartridges you will have two VERY different ballistic effects.  Kinematic Viscosity (resistance of a substance to flow if a liquid, or yield if a solid) tells us that human tissue is primarily viscous - our body opposes penetration in direct proportion to the projectiles velocity.  The slower the projectile, the less the resistance.  The faster the projectile the more the resistance.  High velocity impact creates hydraulic action in the body that disrupts the blood vessels and other body parts that are yielding.  This action affects areas of the body that are not directly impacted by the projectile.  The higher the velocity and abrupt shock also increase nerve and bone damage.  This effect is strongest at velocities at and over 3000 feet per second.  Higher velocity = more explosive effect.  If you shot both of our projectiles from an M4 and an M16 into an area that is primarily water like (the brain) you would have the same desired effect (enemy terminated) but the projectile with the higher velocity WILL do more damage.  This is due to the fact that the hydraulic action created by the higher velocity projectile moving through the brain cavity will cause an expansion more violent than a lower velocity projectile.  That to me is a profound Ballistic difference.  The "explosive" effects between the two projectiles at the two different velocities would likely produce the same end results if your target was a 6 foot tall 200 pound person and they were shot in the stomach.  The body will "spring" back fairly quickly from cavitation and when it does that the depth and width of the wound channel are fairly similar from both weapon platforms leading generally everyone to believe we didn't loose any "ballistic" performance when in fact we have lost it.  Speaking about the Performance of the two weapons systems, they are very simliar.  We can take down targets with them, they are accurate at our familiar distances, etc. 

Very well explained SGT, but in the weapons explained we have lost the terminal effect of the original M-16, the 1:7 twist rates were extremely terminal compared to the lower twist rates. A 1:9 rate will not tumble as much as the 1:7. The 1:7 is very stable in air, but becomes very unstable in denser mediums such as flesh and muscle, the wound channels and the energy is very destructive with the bullet tumbling.Thank you for your input.  

SPC Basom, There is actually a ballance between the barrel length and the twist rate. If you have, for example a 24" barrel the more velocity you have, they more you have to spin the projectile. It's like throwing a football. If you throw it a long way, you need a good tight spiral to make it more accurate. If you want short throw it can wabble quite a bit and still be fairly accurate.

That being said, the 1:9 doesn't wabble. My AR15 (Windham Weaponry) has a 1:9 twist and I can hit whatever I want with Iron sights out to around 150yards (that's the longest range I have tried thus far) I use mixed ammo from 55 grain to 70 grain bullets. HP's to FMJ's It's a little different each round but it's close enough to do the job.

This does litterally nothing to the terminal ballistics unless it drops below the speed of sound. Then it will tumble regardless.

1:7 is only that way because of bullet length. At the end of the day, what matters in "stopping power" is ft-lbs of energy at the target distance. It doesn't matter if it's spinning 1:7 or 1:9 as long as it hits above mach 1.

Also, the newer ammunition has a "penetetrator" that also acts as a top heavy weight so to speak. As soon as it hits, that extra weight at the base is going to keep going causing the bullet to yaw. Unless it hits below the sound barrior and even then it will do it anyway.

That, however has nothing to do with "stopping power" and more to do with damage.

The older rifles had 1/14in twist rates in the barrel making the projectile inherently unstable and that's where you would see the projectile "tumble" throughout a target.  Todays rifles typically have a 1/7in twist which causes the projectile to be much more stable and doing so will increase your firing distances past 500 meters, but of course lowers the lethality when striking tissue.  HOWEVER both rifles (considering barrel lengths) still have a range where the projectile will fragment and cause much more tissue damage.

I did see the M-16A1s with 1:7 twist rates, this higher spin rate made them stable in air and very unstable in flesh. The 1:7 rate was condemned by the Swiss Red Cross as inhumane. That's what I believe happened Sgt. Thank you for your input.

The M16A1s had a 1:12 twist.  But twist rate does not by itself affect lethality.  The faster twist in the A2s and beyond was needed to properly stabilize the longer (and ergo heavier) bullets used in the newer rifles.  Without a fast twist rate, the rifle would be woefully inaccurate.  That is the exact reason you can't shoot M855 out of the old M16A1 and expect any kind of accuracy. 

A better indicator of "lethality" is velocity.  It is velocity that dictates whether the bullet will fragment when it goes through it's yaw cycle (and ALL spritzer shaped bullets will yaw eventually when it either hits anything denser than air or destabilizes).  The general threshold for fragmentation for the M855 round is about 2500fps.  Out of a 20" M16 barrel, that is out to about 300m.  Out of a 14.5" M4 that is only out to about 100m.  But that is what the weapon was designed to do.  The M4 WAS NOT...let me say that again...WAS NOT ever intended to be a general issue Infantry combat rifle.  It was originally intended to be issued to soldiers who's primary job was anything but being a trigger puller (ie medics, vehicle drivers, pilots, officers and NCOs) but needed something more than a pistol.  You are trying to make a carbine do the job of a rifle and that is why you are seeing a drop in performance at longer distances.